1. Field of the Invention
The invention relates to sulphonate-functional polyvinyl acetals, a process for their preparation, and their use.
2. Description of the Related Art
The preparation of polyvinyl acetals from the corresponding polyvinyl alcohols via polymer-analogous reaction with the corresponding aldehydes has been known since as early as 1924, and in the period since then a wide variety of aldehydes has been used to prepare the corresponding polyvinyl acetals. Polyvinyl acetals are prepared in a three-stage process (polyvinyl acetate→polyvinyl alcohol→polyvinyl acetal), the result being products which still contain vinyl alcohol units and vinyl acetate units, alongside vinyl acetal groups. Polyvinyl formal, polyvinyl acetal and polyvinyl butyral have achieved particular commercial importance.
Polyvinyl butyrals are also used as binders in lacquers and specifically in printing inks, one of the reasons for this being their good pigment binding power. In this application, the organic solutions of the polyvinyl butyrals are required to have minimum solution viscosity so that they can be used to produce inks with high solids content and with maximum binder content.
The solution viscosity of polyvinyl butyral is controlled in the prior art by way of the molecular weight, by using, as starting material for the preparation of the polyvinyl butyral, polyvinyl acetates with low molecular weight or low degree of polymerization. By way of example, DE-B 2643076 (U.S. Pat. No. 4,130,539) describes the use of low-molecular-weight polyvinyl butyrals as carrier material in pigment preparations for printing inks. A disadvantage here is that considerable problems arise during the preparation of low-molecular-weight polyvinyl butyrals via acetalization of low-molecular-weight polyvinyl alcohols, because solutions of low-molecular-weight, completely hydrolysed polyvinyl alcohols tend, inter alia, to gel, and therefore have to be handled at temperatures of at least 50° C. EP 923610 B1 proposes, in order to obtain low solution viscosity, preparing the polyvinyl acetals from copolymers of vinyl acetate and 1-alkylvinyl acetate.